Niobium-containing zeolites

ABSTRACT

Molecular sieve zeolites containing niobium isomorphously substituted in their framework lattice are obtained by hydrothermal crystallization using quaternary ammonium templates. The zeolites are useful catalysts, particularly for the oxidation of hydrocarbons such as olefins.

FIELD OF THE INVENTION

This invention relates to siliceous niobium-containing crystallinecompositions wherein niobium is substituted in the framework of amicroporous molecular sieve. The zeolites usefully catalyze theoxidation of organic substrates, including epoxidation of olefins usinghydrogen peroxide.

BACKGROUND OF THE INVENTION

Zeolites are crystalline tectosilicates. Their structures typicallyconsist of assemblies of TO₄ tetrahedra forming a three dimensionalframework by sharing of the oxygen atoms. In zeolites of thealuminosilicate type, which are the most common, T representstetravalent silicon as well as trivalent aluminum. The cavities andchannels of this framework are of molecular dimension and collectcations, compensating the charge deficit associated with the presence oftrivalent aluminum in the tetrahedra. Trivalent elements such as galliumor boron can be substituted for the aluminum.

In general, the composition of zeolites can be represented by theempirical formula M_(2/n) O·Y₂ O₃ ·xZ0₂ in the dehydrated and calcinedstate. Z and Y respectively represent the tetravalent and trivalentelements of the TO₄ tetrahedra. Typically, Z is Si and Y is Al. Mrepresents an electropositive element of valence n such as an alkali oralkaline earth metal, constituting the compensating cations. The valueof x may range in theory from 2 to infinity, in which case the zeoliteis a silica (silicalite).

Each type of zeolite has a distinct porous structure. Examples of typesof zeolites having different three dimensional arrangements of theirframework elements include ZSM-5 (MFI), zeolite beta, zeolite A, and soforth. The variation in pore size and shape from one type to anothercauses changes in the absorbent properties. Only molecules with certainsizes and shapes are capable of entering the pores of a particularzeolite. The chemical composition along with, in particular, the natureof the elements present in the TO₄ tetrahedra and the nature of theexchangeable compensating cations are also important factors affectingthe absorptive selectivity and especially the catalytic properties ofthese products. Zeolites are consequently used as catalysts or catalystsupports in the cracking, reforming, and modification of hydrocarbonsand the synthesis of various organic compounds.

For example, molecular sieves containing titaniun atoms isomorphouslysubstituted for a portion of the silicon atoms in their frameworklattice have in recent years been found to be highly active and usefulcatalysts for the epoxidation of olefins using hydrogen peroxide. See,for example, U.S. Pat. Nos. 4,833,260 and 5,453,511.

The substitution of different transition metals into the frameworkstructures of molecular sieves is not straightforward, however, andoften can only be successfully accomplished through very carefulselection of reactants and reaction conditions. The preparation oftransition metal-containing molecular sieves remains a highly uncertainand unpredictable art. For instance, while European Pat. Pub. No. 77,522claimed the preparation of titano-aluminosilicates having a pentasil(ZSM-5) structure, later workers (Skeels et al., U.S. Pat. No.5,098,687) demonstrated that the titanium atoms in the materialsobtained were not actually present in the form of framework tetrahedraloxides.

To date, there have been few reports of niobium-containing molecularsieves in the literature. European Pat. Pub. No. 178,723 disclosedcatalyst compositions for catalytic cracking of hydrocarbons comprisedof porous matrix material containing crystalline aluminosolicates and aniobium component. However, it is clear from the description providedthat the niobium in said catalyst compositions is merely impregnated orsupported on the crystalline aluminosilicate and is not incorporated inthe framework structure of the zeolite itself. Japanese Kokai JP04-349,115 teaches crystalline silicates having the chemical compositionin dehydrated form:

    (0.1-2.0)R.sub.2.0)R.sub.2/n O·[aM.sub.2 O.sub.3 ·bAl.sub.2 O.sub.3 ]·ySiO.sub.2

wherein R is ≧1 monovalent or divalent ions, n is the valence of R, Mcan be Nb or another metal, a+b=1,a>1,b≧0,y≧12. Such materials thus mustnecessarily contain relatively high levels of exchangeable cations(e.g., alkali metal oxide or alkaline earth metal oxide cations). Thereis no suggestion that such crystalline silicates could effectivelycatalyze the epoxidation of olefins.

SUMMARY OF THE INVENTION

We have now unexpectedly discovered that it is possible to prepare acrystalline siliceous molecular sieve zeolite wherein niobium isisomorphously substituted for silica in the framework. In one embodimentof the invention, the zeolite has a silicalite (MFI) morphology. Theframework niobium is retained during calcination. The calcined zeolitehas been found to be active as an olefin epoxidation catalyst usingaqueous hydrogen peroxide. This was quite surprising in view of the factthat although a number of different elements, including Al, Sn, V, Cr,Fe, Ga, and In have previously been reported to be isomorphouslysubstituted in the framework of an all-silica ZSM-5, nometallosilicalite other than titanium silicalite has shown anysignificant epoxidation activity. Moreover, heterogeneous niobiumcompounds in general have heretofore been regarded as having little orno utility as olefin epoxidation catalysts. For instance, Sheldon [J.Mol. Cat. 7, 107-126 (1980)] reported that Nb₂ O₅ gave only 9% t-butylhydroperoxide conversion and 0% selectivity to 1-octene; oxide after 4.5hours at 110° C. Only 5% epoxide selectivity was obtained using aniobium catalyst supported on silica.

DETAILED DESCRIPTION OF THE INVENTION

One objective of the present invention is to provide a new zeolite basedon silica and niobium oxide.

Another objective of the invention is to synthesize the aforementionedzeolite according to a process wherein sources of silica and niobiumoxide are combined and subjected to hydrothermal crystallization in thepresence of a quaternary ammonium compound capable of acting as atemplate or structure directing agent. Such process incorporates niobiuminto the zeolite framework and leads to the production of microporouszeolite crystals in high yield having controlled pore dimensions and ahigh degree of crystallinity.

The zeolites of the present invention desirably correspond in theiranhydrous calcined state to the following general formula (expressed inmolar ratios):

    xNb.sub.2 O.sub.5 ·(1-x)SiO.sub.2

wherein x is from about 0.0005 to about 0.1. The Si/Nb molar ratio maythus be from about 4.5 to about 1000. Such zeolites are distinguishablefrom previously known niobium-containing molecular sieves by their lowexchangeable cation levels. That is, the zeolites of the invention arecharacterized by the substantial absence of cations such as alkali metaland alkaline earth metal cations. In this context, "substantial absence"means that less than a total of 1000 ppm of such cations are present inthe zeolite. The invention is capable of providing zeolites containingless than 100 ppm alkali metals and alkaline earth metals in total. Inaddition, little or no aluminum is present; the zeolites preferablycontain less than 500 ppm Al. In a preferred embodiment of theinvention, the zeolite comprises niobium in an amount effective tocatalyze the epoxidation of olefins.

In a preferred embodiment of the invention, zeolites belonging to thepentasil family and having an MFI (ZSM-5) type structure are obtained.The zeolites have a crystallinity of at least 75%, as measured by XRD;the synthetic methods described herein are capable of providingcrystallinities of 95% or greater. The zeolites of the present inventionhave a microporous structure and desirably have hexane adsorption valuesin the range of from about 0.17 to 0.20 cc/g.

If at least a portion of the niobium is incorporated into the latticeframework of the zeolite in the +5 oxidation state, the resultingpositively changed framework may develop anion exchange capacity.Inorganic or organic anionic species can then be introduced into thezeolite by ion exchange. The identity and concentration of the anionicspecies may be manipulated so as to desirably alter the sorptioncapacity and catalytic activity of the zeolite.

Preferred methods for synthesizing the niobium-containing molecularsieves which are the subject of this application involve the use of aquaternary ammonium species. Without wishing to be bound by theory, itis believed that the quaternary ammonium species may function as atemplate for directing the assemblage of the required zeolite latticeframework from reactants which serve as sources of Si and Nb, but couldalso be accomplishing the desired synthetic result by acting as a bufferor structure-directing agent. The use of the term "template" herein isnot meant to indicate that the quaternary ammonium species is in factparticipating in a templating mechanism. The use of a quaternaryammonium species to prepare the catalysts of the present invention isadvantageous since such a method tends to furnish molecular sieves whichhave low acidity, have little or no extra-framework Nb, and contain fewdefect sites.

Suitable quaternary ammonium species include, but are not limited to,compounds having the formula: ##STR1## wherein R¹, R², R³, and R⁴ arethe same or different and represent a linear or branched alkyl groupwith 1 to 6 carbon atoms. Preferably, the substituents on N are propylor butyl groups. X may be halide or, more preferably, hydroxide. Themost preferred quaternary ammonium species is tetrapropylammoniumhydroxide. The morphology of the zeolite produced may be varied by theuse of different quaternary ammonium species.

The niobium-containing molecular sieve of this invention may be preparedusing the quaternary ammonium species in a solution-type synthesis. Thismethod comprises forming a mixture, preferably in solution, of ahydrolyzable silicon compound, a hydrolyzable niobium compound, and thequaternary ammonium species, and subjecting said mixture to hydrothermaltreatment at a temperature of from 100° to 200° C. (more preferably,120° to 180° C.) for a time effective to form the crystallineniobium-containing molecular sieve. Such hydrothermal treatment is mostpreferably conducted in an aqueous medium (which may, in addition towater, contain a water-miscible organic solvent such as an alcohol)under conditions such that hydrolysis and/or mineralization of thesilicon and niobium compounds is achieved. The process may be catalyzedby base, if so desired.

The hydrolyzable silicon compound may be any substance capable offunctioning as a source of SiO₂ (silica) including, for example,amorphous or fumed silica or, more preferably, a tetraalkoxysilane suchas tetraethyl orthosilicate or the like. Suitable hydrolyzable niobiumcompounds are those species which serve as a source of Nb₂ O₅ (niobiumoxide) such as niobium halide (e.g., NbCl₅) or, more preferably aniobium alkoxide such as niobium, isopropoxide or the like.

The starting reagents may, for example, may be selected to provide thefollowing preferred molar ratios:

    ______________________________________                                               SiO.sub.2 /Nb.sub.2 O.sub.5                                                                  5-2000                                                         OH.sup.- /SiO.sub.2                                                                          0.1-2.0                                                        H.sub.2 O/SiO.sub.2                                                                          20-200                                                         Q.sup.+ /SiO.sub.2                                                                           0.1-2.0                                                 ______________________________________                                    

wherein Q is the cation associated with the quaternary ammonium species.A suitable preferred procedure for accomplishing formation of themixture is as follows: partial hydrolysis of the hydrolyzable siliconcompound is first carried out by reacting said compound with watercontaining a portion of the quaternary ammonium species (in hydroxideform). The partial hydrolysis product thereby obtained is then combinedwith the hydrolyzable niobium compound (and, optionally, an additionalamount of the hydrolyzable silicon compound). The remaining amount ofthe quaternary ammonium species is thereafter added to yield a precursorgel or solution. Any volatile co-products generated as a result ofhydrolysis (such as, for example, alcohols where the hydrolyzablesilicon compound is a tetraalkyl orthosilicate or where the hydrolyzableniobium compound is a niobium alkoxide) may, if desired, be removed byany suitable means such as distillation or evaporation prior tohydrothermal treatment. The hydrothermal treatment is advantageouslyperformed in an autoclave or other closed reactor under autogenouspressure. Typically, a period of from 3 to 20 days is sufficient to formthe niobium-containing molecular sieve in crystalline, precipitatedform. Such as-synthesized crystals, which will generally contain thequaternary ammonium species template, may be separated from the motherliquor by suitable means such as filtration, decantation, orcentrifugation, washed with a suitable liquid medium such as water, thendried.

The crystalline product obtained by the above-described techniques may,if so desired, be calcined in air or the like at a temperature in excessof 400° C. in order to remove any template still present within thepores of the molecular sieve. The calcined molecular sieve may becontacted with hydrogen peroxide prior to use as a catalyst in order toincrease its catalytic activity.

The niobium-containing molecular sieve of this invention may also besynthesized by adaptation of standard zeolite preparation techniquessuch as, for example, co-gel impregnation with template followed byhydrothermal treatment or dealumination of an aluminosilicate zeolitefollowed by reaction with a volatile niobium compound such as NbCl₅ toinsert Nb into the framework vacancies created by dealumination.

In the epoxidation process of this invention, an olefin is contactedwith hydrogen peroxide (or a substance capable of producing hydrogenperoxide under the reaction conditions)in the presence of acatalytically effective amount of the niobium-containing molecular sievezeolite described hereinabove.

The amount of catalyst employed to epoxidize an olefin is not critical,but should be sufficient so as to substantially accomplish the desiredreaction in a practicably short period of time. The optimum quantity ofcatalyst will depend upon a number of factors including reactiontemperature, olefin reactivity and concentration, oxidizing agentconcentration, type and concentration of organic solvent as well ascatalyst activity. Typically, however, in a batch type epoxidation, theamount of catalyst will be from 0.001 to 10 grams per mole of olefin. Ina fixed bed system, the optimum bed (typically, from about 1 to 100moles oxidizing agent per kilogram of catalyst per hour). Theconcentration of niobium in the total epoxidation reaction mixture willgenerally be from about 10 to 10,000 ppm.

The catalyst may be utilized in powder, pellet, microspheric,monolithic, extruded, or any other suitable physical form. The use of abinder (co-gel) or support in combination with the niobium-containingmolecular sieve may be advantageous. Supported or bound catalyst may beprepared by the methods known in the art to be effective for zeolitecatalysts in general.

Illustrative binder and supports (which preferably are non-acidic incharacter) include silica, alumina, titania, silica-alumina,silica-titania, silica-thoria, silica-magnesia, silica-zironia,silica-beryllia, and ternary compositions of silica with otherrefractory oxides. Also useful are clays such as montmorillonites,kaolins, bentonites, halloysites, dickites, nacrites, and anaxites. Theproportion of niobium-containing molecular sieve to binder or supportmay range from 99:1 to 1:99 but preferably is from 5:95 to 80:20.

The olefin substrate epoxidized in the process of this invention may beany organic compound having at least one ethylenically unsaturatedfunctional group (i.e., a carbon-carbon double bond) and may be acyclic, branched or straight chain olefin. The olefin may contain arylgroups (e.g., phenyl, naphthyl). Preferably, the olefin is aliphatic incharacter and contains from 2 to 30 carbon atoms (i.e., a C₂ -C₃₀olefin). The use of light (low-boiling) C₂ to C₁₀ mon-olefins isespecially advantageous. More than one carbon-carbon double bond may bepresent in the olefin; dienes, trienes, and other polyunsaturatedsubstrates thus may be used. The double bond may be in a terminal orinternal position in the olefin or may alternatively form part of acyclic structure (as in cyclohexene, for example). Other examples ofsuitable substrates include unsaturated fatty acids or fatty acidderivatives such as esters or glycerides and oligomeric or polymericunsaturated compounds such as polybutadiene. Benzylic and styrenicolefins may also be epoxidized, although the epoxides of certainstyrenic olefins such as styrene may further react or isomerize underthe conditions utilized in the present invention to form aldehydes andthe like.

The olefin may contain substituents other than hydrocarbon substituentssuch as halide, carboxylic acid, ether, hydroxy, thiol, nitro, cyano,ketone, acyl ester, anhydride, amino, and the like.

Exemplary olefins suitable for use in the process of this inventioninclude ethylene, propylene, the butenes (e.g., 1,2-butene, 2,3-butene,isobutylene), butadiene, the pentenes, isoprene, 1-hexene, 3-hexene,1-heptene, 1-octene, diisobutylene, 1-nonene, 1-tetradecene,pentamyrecene, camphene, 1-undecene, 1-dodecene, 1-tridecene,1-tetradecene, 1-pentadecene, 1-hexaddecene, 1-heptadecene,1-octadecene, 1-nonadecene, 1-eicosene, the trimers and tetramers ofpropylene, styrene (and other vinyl aromatic substrates) polybutadiene,polyisoprene, cyclopentene, cyclohexene, cycloheptene, cyclooctene,cyclooctadiene, cyclododecene, cyclododecetriene, dicyclopentadiene,methylenecyclopropane, methylenecyclopentane, methylenecyclohexane,vinyl cyclohexane, vinyl cyclohexene, methallyl ketone, allyl chloride,allyl bromide, acrylic acid, methacrylic acid, crotonic acid, vinylacetic acid, crotyl chloride, methallyl chloride, the dichlorobutenes,allyl alcohol, allyl carbonate, allyl acetate, alkyl acrylates andmethacryltaes, diallyl maleate, diallyl phthalate, unsaturatedtriglycerides such as soybean oil, and unsaturated fatty acids, such asoleic acid, linolenic acid, linoleic acid, erucic acid, palmitoleicacid, and riconoleic acid and their esters (including mono-, di-, andtriglyceride esters) and the like.

Mixtures of olefins may be epoxidized and the resulting mixtures ofepoxides either employed in mixed form or separated into the differentcomponent epoxides.

The process of this invention is especially useful for the epoxidationof C₂ -C₃₀ olefins having the general structure ##STR2## wherein R¹, R²,R³, and R⁴ are the same or different and are selected from the groupconsisting of hydrogen and C₁ -C₂₀ alkyl.

The oxidizing agent employed in the process of this invention may be ahydrogen peroxide source such as hydrogen peroxide (H₂ O₂) or a compoundwhich under the epoxidation reaction conditions is capable of generatingor liberating hydrogen peroxide.

The amount of oxidizing agent relative to the amount of olefins notcritical, but most suitably the molar ratio of oxidizing agent:olefin isfrom 100:1 to 1:100 when the olefin contains one ethylenicallyunsaturated group. The molar ratio of ethylenically unsaturated groupsin the olefin substrate to oxidizing agent is more preferably in therange of from 1:10 to 10:1. One equivalent of oxidizing agent istheoretically required to oxidize one equivalent of a mono-unsaturatedolefin substrate, but it may be desirable to employ an excess of onereactant to optimize selectivity to the epoxide.

Although the hydrogen peroxide which may be utilized as the oxidizingagent may be derived from any suitable source, a distinct practicaladvantage of the process of this invention is that the hydrogen peroxidemay be obtained by contacting a secondary alcohol such as alpha-methylbenzyl alcohol, isopropyl alcohol, 2-butanol, or cyclohexanol withmolecular oxygen under conditions effective to form an oxidant mixturecomprised of secondary alcohol and hydrogen peroxide (and/or hydrogenperoxide precursors). Typically, such an oxidant mixture will alsocontain a ketone such as acetophenone, acetone, or cyclohexanonecorresponding to the secondary alcohol (i.e., having the same carbonskeleton), minor amounts of water, and varying amounts of other activeoxygen species such as organic hydroperoxides. Molecular oxygenoxidation of anthrahydroquinone, alkyl-substituted anthrahydroquinones,or water-soluble anthrahydroquinone species may also be employed togenerate the hydrogen peroxide oxidant. The hydrogen peroxide may begenerated in situ immediately prior to or simultaneous with epoxidation.

If desired, a solvent may additionally be present during the epoxidationprocess of this invention in order to dissolve the reactants other thanthe niobium-containing molecular sieve catalyst, to provide bettertemperature control, or to favorably influence the epoxidation rates andselectivities. The solvent, if present, may comprise from 1 to 99 weightpercent of the total epoxidation reaction mixture and is preferablyselected such that it is a liquid at the epoxidation reactiontemperature. Organic compounds having boiling points at atmosphericpressure of from about 25° C. to 300° C. are generally preferred foruse. Excess olefin may serve as a solvent or diluent. Illustrativeexample of other suitable solvents include, but are not limited to,ketones (e.g., acetone, methyl ethyl ketone, acetophenone), ethers(e.g., tetrahydrofuran, butyl ether), nitriles (e.g., acetonitrile),aliphatic and aromatic hydrocarbons, halogenated hydrocarbons, andalcohols (e.g., methanol, ethanol, isopropyl alcohol, t-butyl,alpha-methyl benzyl alcohol, cyclohexanol, trifluoroethanol). More thanone type of solvent may be utilized. Water may also be employed as asolvent or diluent; surprisingly, the process of the invention proceedswith minimal hydrolysis even when a significant quantity of water ispresent in the epoxidation reaction mixture. Biphasic as well asmonophasic reaction systems thus are possible using the presentinvention.

The reaction temperature is not critical, but should be sufficient toaccomplish substantial conversion of the olefin to epoxide within areasonably short period of time. It is generally advantageous to carryout the reaction to achieve as high a conversion of oxidizing agent aspossible, preferably at least 50%, more preferably at least 90% mostpreferably at least 95%, consistent with reasonable selectivities. Theoptimum reaction temperature will be influenced by catalyst activity,olefin and oxidizing agent reactivity, reactant concentrations, and typeof solvent employed, among other factors, but typically will be in arange of from about 0° C. to 150° C. (more preferably, from about 20° C.to 100° C). Reaction or residence times of from about 1 minute to 48hours will typically be appropriate, depending upon the above-identifiedvariables. Although subatmospheric pressures can be employed, thereaction is preferably (especially when the boiling point of the olefinis below the epoxidation reaction temperature) performed at atmosphericpressure or at elevated pressure (typically, between 1 and 100atmospheres). Generally, it will be desirable to pressurize theepoxidation vessel sufficiently maintain the reaction the reactioncomponents as a liquid phase mixture. For example, performing theepoxidation at elevated pressure will increase the solubility of gaseousreactants such as propylene in the solvent and oxidizing agent.

The process of this invention may be carried out in a batch, continuous,or semi-continuous manner using any appropriate type of reaction vesselor apparatus such as a fixed bed, transport bed, fluidized bed, stirredslurry, or CSTR reactor in a monophase or biphase system. Known methodsfor conducting metal-catalyzed epoxidations of olefins using an activeoxygen oxidizing agent will generally also be suitable for use in thisprocess. Thus, the reactants may be combined all at once orsequentially. For example, the oxidizing agent may be addedincrementally to the reaction zone, the oxidizing agent could also begenerated in situ within the same reactor zone where epoxidation istaking place. Once the epoxidation has been carried out to the desireddegree of conversion, the epoxide product may be separated and recoveredfrom the reaction mixture using any appropriate technique such asfractional distillation, extractive distillation, liquid-liquidextraction, crystallization, or the like. After separating from theepoxidation reaction mixture by any suitable method such as filtration,the recovered catalyst may be economically reused in subsequentepoxidations. Where the catalyst is deployed in the form of a fixed bed,the epoxidation product withdrawn as a stream from the epoxidation zonewill be essentially catalyst free with the catalyst being retainedwithin the epoxidation zone. In certain embodiments of the instantprocess where the epoxide is being produced on a continuous basis, itmay be desirable to periodically or constantly regenerate all or aportion of the used niobium-containing molecular sieve catalyst in orderto maintain optimum activity and selectivity. Suitable regenerationtechniques include, for example, treating the catalyst with solventand/or calcining the catalyst. Any unreacted olefin or oxidizing agentmay be similarly separated and recycled. Alternatively, the unreactedoxidizing agent (especially if present at concentrations too low topermit economic recovery) could be thermally or chemically decomposedinto non-peroxy species. In certain embodiments of the process where theoxidizing agent is hydrogen peroxide generated by molecular oxygenoxidation of a secondary alcohol, the crude epoxidation reaction mixturewill also contain a secondary alcohol and ketone corresponding to thesecondary alcohol. After separation of the epoxide from the secondaryalcohol and the corresponding ketone, the ketone may be converted backto secondary alcohol by hydrogenation. For example, the ketone may bereacted with hydrogen in the presence of a transition metalhydrogenation catalyst. Hydrogenation reactions of this type are wellknown to those skilled in the art. The secondary alcohol may also bedehydrated using know methods to yield valuable alkenyl products such asstyrene.

The niobium-containing molecular sieve described herein, in addition tobeing a useful epoxidation catalyst, also has utility as an ionexchanger, a shape-selective separation medium or a catalyst for otherhydrocarbon conversion processes, including, for example: cracking,selectoforming, hydrogenation, dehydrogenation, oligomerization,alkylation, isomerization, dehydration, hydroxylation of olefins oraromatics, alkane oxidation, reforming, disproportionation, methanation,and the like. The molecular sieve of this invention is particularlyuseful for catalyzing the same reactions wherein titanium silicalites(also referred to as titanium silicates) have heretofore been employed.Illustrative applications of this type are as follows:

a) A process for the manufacture of a ketone oxime which comprisesreacting a ketone such as cyclohexanone with ammonia and hydrogenperoxide in the liquid phase at a temperature of from 25° C. to 150° C.in the presence of a catalytically effective amount of theniobium-containing molecular sieve. Reactions of this general type arewell known in the art and suitable conditions for carrying out such asynthetic transformation in the presence of a titanium silicalitecatalyst are described, for example, in U.S. Pat. No. 4,745,221, Roffiaet al., "Cyclohexanone Ammoximation: A Breakthrough in the 6-CaprolactamProduction Process", in New Developments in Selective Oxidation, Centiet al, eds., pp. 43-52 (1990), Roffia et al., "A New Process forCyclohexanonoxime", La Chimica & L'Industria 72, pp. 598-603 (1990),U.S. Pat. No. 4,894,478, U.S. Pat. No. 5,041,652, U.S. Pat. No.4,794,198, Reddy et al., "Ammoximation of Cyclohexanone Over a TitaniumSilicate Molecular Sieve", J. Mol. Cat. 69, 383-392 (1991), EuropeanPat. Pub. No. 4.96,385, European Pat. Pub. No. 384,390, and U.S. Pat.No. 4,968,842, (the teachings of the foregoing publications areincorporated herein by reference in their entirety).

b) A process for oxidizing a paraffinic compound (i.e., a saturatedhydrocarbon) comprising reacting the paraffinic compound at atemperature of from 25° C. to 200° C. with hydrogen peroxide in thepresence of a catalytically effective amount of the niobium-containingmolecular sieve. Reactions of this general type are well known in theart and suitable conditions for carrying out such a synthetictransformation in the presence of a titanium silicalite are described,for example, in Huybrechts et al., Nature 345,240 (1990), Appl. Catal.68, 249 (1991), and Tatsumi et al., J. Chem. Soc. Chem. Commun. 476(1990), Huybrechts et al., Catalysis Letters 8, 237-244 (1991 ), theteachings of which are incorporated herein by reference in theirentirety.

c) A process for hydroxylating an aromatic hydrocarbon (e.g., phenol)comprising reacting the aromatic compound at a temperature of from 50°to 150° C. with hydrogen peroxide in the presence of a catalyticallyeffective amount of the niobium-containing molecular sieve to form aphenolic compound (e.g., cresol). Reactions of this general type arewell known in the art and suitable conditions for carrying out such asynthetic transformation in the presence of a titanium silicalitecatalyst are described, for example, in U.S. Pat. No. 4,396,783, Romanoet al., "Selective Oxidation with Ti-silicalite", La Chimica L'Industria72, 610-616 (1990), Reddy et al., Applied Catalysis 58, L1-L4 (1990).

d) A process for isomerizing an aryl-substituted epoxide to thecorresponding beta-phenyl aldehyde comprising contacting thearyl-substituted epoxide with a catalytically effective amount of theniobium-containing molecular sieve at a temperature of from 25° C. to150° C. See, for example, U.S. Pat. No. 4,495,371 (incorporated hereinby reference in its entirety).

e) A process for oxidizing a vinyl benzene compound to the correspondingbeta-phenyl aldehyde comprising reacting the vinyl benzene compound withhydrogen peroxide at a temperature of from 20° C. to 150° C. in thepresence of the niobium-containing molecular sieve. See, for an exampleof the use of titanium silicalite for such a transformation, U.S. Pat.No. 4,609,765 (incorporated herein by reference in its entirety).

f) A process for synthesizing an N, N-dialkyl hydroxylamine comprisingreacting the corresponding secondary dialkyl amine with hydrogenperoxide in the presence of the niobium-containing molecular sieve. See,for an example of the use of titanium silicalite for such atransformation, U.S. Pat. No. 4,918,194 (incorporated herein byreference in its entirety).

g) A process for oxidizing an aliphatic alcohol comprising reacting thealiphatic alcohol with hydrogen peroxide in the presence of theniobium-containing molecular sieve at a temperature of from 25° C. to150° C. to form the corresponding ketone or aldehyde of said aliphaticalcohol. See, for an example of the use of titanium silicalite for sucha transformation, U.S. Pat. No. 4,480,135 (incorporated herein byreference in its entirety).

h) A process for synthesizing a glycol monoalkyl ether comprisingreacting an olefin, an aliphatic alcohol, and hydrogen peroxide in thepresence of the niobium-containing molecular sieve at a temperature offrom 25° C. to 150° C. See, for an example of the use of titaniumsilicalite for such a transformation, U.S. Pat. No. 4,476,327(incorporated herein by reference in its entirety).

From the foregoing description, one skilled in the art can readilyascertain the essential characteristics of this invention, and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt to its various usages,conditions, and embodiments.

EXAMPLES

A crystalline siliceous molecular sieve zeolite containing frameworkniobium in accordance with the present invention is prepared as follows.To a 250 mL plastic beaker was charged 40 g (0.192 mole)tetraethylorthosilicate. To this was added dropwise 33.2 g of 40% aqueoustetrapropylammonium hydroxide solution (containing 0.065 mole TPAOH).The resulting clear solution was then stirred for one hour at roomtemperature. After this time, 6.27 g of a 10% niobium isopropoxidesolution in isopropanol (containing 0.0016 mole niobium isopropoxide)was added dropwise. The solution, which remained clear, was stirred foran additional hour. After this time, an additional 4.2 g of 40% aqueousTPAOH was added (0.0083 mol TPAOH; 0.073 mole TPAOH total) together with13 g deionized water. The solution was stirred one hour and then heatedin a water bath at 70° C. with gentle nitrogen sparging to remove thealcohols. Over the course of one hour, an additional 14 g deionizedwater was added in aliquots. After this time the clear solution wasloaded into a Teflon-lined Parr reactor and heated statically at 175° C.for 12 days. The white solids which formed were recovered by filtration,washed well with hot water, and dried at 120° C. overnight.

Powder x-ray diffraction analysis of the as-synthesized solids confirmeda crystalline MFI structure. The powder XRD spectrum was substantiallyidentical to that of TS-1 titanium silicalite, except that the peakswere shifted to greater d-spacing as would be expected for a largerframework-substituted atom. The IR spectrum exhibited a peak at 952 cm⁻¹having an intensity one-half that of the 800cm⁻¹ peak. Thermogravimetricanalysis indicated that a 14% weight loss took place above 250° C.,corresponding to the loss of the organic template initially trappedwithin the zeolite structure.

Calcination of the as-synthesized solids at 550° C. for 6 hours in dryair did not result in any significant loss of crystallinity (as measuredby x-ray diffraction analysis). Elemental analysis of the calcinedzeolite yielded the following results: 46% Si, 1.7% Nb, <100 ppm Al,corresponding to a Si/Nb molar ratio of 90. The hexane adsorption valuewas 0.183 cc/g, a void volume equivalent to that of TS-1 titaniumsilicalite and consistent with framework substitution of niobium in thesilicalite.

The calcined niobium silicalite exhibited activity as a selective olefinepoxidation catalyst. After 3 days at room temperature, 1-hexene wasepoxidized with aqueous hydrogen peroxide in acetonitrile (8 mlacetonitrile, 15 mmol olefin, 4.2 mmol H₂ O₂, 0.15 catalyst)to 15%conversion and selectivity to epoxide of 95% based on hydrogen peroxide.No hexanediol or other by-products were observed, suggesting thatniobium silicalite has little or no inherent Lewis or Bronsted acidity.In contrast, unnuetralized titanium silicalite (TS-1) typically producessignificant amounts of glycol and/or glycol ether by-products undersimilar epoxidation conditions due to its higher acidity. After fivedays of reaction with the niobium silicalite, H₂ O₂ conversion increasedto 25%; epoxide selectivity remained >95%. This represents a totalturnover number of about 40 based on niobium. Additional reaction timedid not lead to conversions higher than 25%.

Initial activity was improved by first exposing the catalyst to hydrogenperoxide for 24 hours at 25° C. and then adding olefin. After 12 hoursat room temperature under the same reaction conditions as describedhereinabove, the hydrogen peroxide conversion was 11%. After 72 hours,the conversion had reached 76%. Epoxide selectivity had dropped to 32%,however, corresponding again to 40 total turnovers based on niobium.

We claim:
 1. A method of making a crystalline siliceous molecular sievezeolite having a zeolite framework structure isomorphous with ZSM-5,wherein niobium is substituted in said zeolite framework structure, andcharacterized by the substantial absence of exchangeable cations andaluminum, said method comprising:(a) partially hydrolyzing atetraalkoxysilane by reacting the tetraalkoxysilane with watercontaining a first portion of a quaternary ammonium species in hydroxideform to obtain a partial hydrolysis product; (b) combining the partialhydrolysis product with a hydrolyzable niobium compound and a secondportion of the quaternary ammonium species to yield a precursor product;and (c) subjecting the precursor product to hydrothermal treatment at atemperature of from 100° C. to 200° C. for a time effective to formas-synthesized crystals of the crystalline siliceous molecular sievezeolite.
 2. The method of claim 1 wherein the tetraalkoxysilane istetraethyl orthosilicate.
 3. The method of claim 1 wherein thehydrolyzable niobium compound is a niobium alkoxide.
 4. The method ofclaim 1 wherein the quaternary ammonium species is tetrapropylammoniumhydroxide.
 5. The method of claim 1 wherein an additional portion of thetetraalkoxysilane is combined with the partial hydrolysis product instep (b).
 6. The method of claim 1 wherein the precursor productcomprises volatile alcohols, said volatile alcohols being removed fromthe precursor product prior to step (c).
 7. The method of claim 1wherein the tetraalkoxysilane, quaternary ammonium species, hydrolyzableniobium compound, and water are present in amounts sufficient to providethe following molar ratios:

    ______________________________________                                               SiO.sub.2 /Nb.sub.2 O.sub.5                                                                  5-2000                                                         OH.sup.- /SiO.sub.2                                                                          0.1-2.0                                                        H.sub.2 O/SiO.sub.2                                                                          20-200                                                         Q.sup.+ /SiO.sub.2                                                                           0.1-2.0                                                 ______________________________________                                    

wherein Q is the cation associated with the quaternary ammonium species.8. The method of claim 1 comprising an additional step wherein theas-synthesized crystals are calcined.
 9. The method of claim 1 whereinthe as-synthesized crystals are calcined and subsequently contacted withhydrogen peroxide.
 10. A method of making a crystalline siliceousmolecular sieve zeolite having a zeolite framework structure isomorphouswith ZSM-5, wherein niobium is substituted in the zeolite frameworkstructure, and characterized by the substantial absence of exchangeablecations and aluminum, said method comprising(a) partially hydrolyzing atetraalkoxysilane by reacting the tetraalkoxysilane with watercontaining a first portion of a quaternary ammonium species having thegeneral structure ##STR3## wherein R¹, R², R³ and R⁴ are the same ordifferent and each represent a linear or branched alkyl group having 1to 6 carbon atoms, to obtain a partial hydrolysis product; (b) combiningthe partial hydrolysis product with a niobium alkoxide and a secondportion of the quaternary ammonium species to yield a precursor product;and (c) subjecting the precursor product to hydrothermal treatment at atemperature of from 100° C. to 200° C. for a time effective to formas-synthesized crystals of the crystalline siliceous molecular sievezeolite.
 11. The method of claim 10 wherein the tetraalkoxysilane istetraethyl orthosilicate.
 12. The method of claim 10 wherein the niobiumalkoxide is niobium isopropoxide.
 13. The method of claim 10 wherein thequaternary ammonium species is tetrapropylammonium hydroxide.
 14. Themethod of claim 10 wherein an additional portion of the tetraalkoxysilane is combined with the partial hydrolysis product in step (b). 15.The method of claim 10 wherein the precursor product comprises volatilealcohols, said volatile alcohols being removed from the precursorproduct prior to step (c).
 16. The method of claim 10 wherein thetetraalkoxysilane, quaternary ammonium species, hydrolyzable niobiumcompound, and water are present in amounts sufficient to provide thefollowing molar ratios:

    ______________________________________                                               SiO.sub.2 /Nb.sub.2 O.sub.5                                                                  5-2000                                                         OH.sup.- /SiO.sub.2                                                                          0.1-2.0                                                        H.sub.2 O/SiO.sub.2                                                                          20-200                                                         Q.sup.+ /SiO.sub.2                                                                           0.1-2.0                                                 ______________________________________                                    

wherein Q is the cation associated with the quaternary ammonium species.17. The method of claim 10 comprising an additional step wherein theas-synthesized crystals are calcined.
 18. The method of claim 10 whereinthe as-synthesized crystals are calcined and subsequently contacted withhydrogen peroxide.
 19. The method of claim 10 wherein the crystallinesiliceous molecular sieve zeolite in an anhydrous calcined statecorresponds to the general formula

    xNb.sub.2 O.sub.5 ·(1-x)SiO.sub.2

wherein x is from about 0.0005 to about 0.1.